Allow checkpoint-only version artifact directories

[anth-volk]

Summary

Fixes #179.

• Allow an explicit version artifact directory to be reused when its existing contents are limited to the configured PE-US rebuild checkpoint trees for the same run.
• Preserve the existing collision behavior for unrelated contents, unconfigured checkpoint contents, missing checkpoint configuration, and non-directory version paths.
• Forward stage_runtime_writer through save_versioned_us_microplex_artifacts() to match the existing _finalize_versioned_build_artifacts() call path.

Notes on Scope

The checkpoint-only directory allowance is the direct #179 fix. While validating the full versioned-artifacts test file, current main also exposed an adjacent finalization mismatch: _finalize_versioned_build_artifacts() already passed stage_runtime_writer, but save_versioned_us_microplex_artifacts() did not accept or forward it. This PR includes that small pass-through so the live stage-runtime path remains consistent.

Tests

• ruff check src/microplex_us/pipelines/versioned_artifacts.py tests/pipelines/test_versioned_artifacts.py
• git diff --check
• PYTHONPATH=/Users/administrator/Documents/PolicyEngine/microplex-us/src PYTHONDONTWRITEBYTECODE=1 /Users/administrator/Documents/PolicyEngine/worktrees/microplex-us/fix-pe-rebuild-smoke-issues/.venv/bin/python -m pytest -q -p no:cacheprovider tests/pipelines/test_versioned_artifacts.py -k 'configured_checkpoint or unrelated_existing or unconfigured_checkpoint or existing_version_file'
• PYTHONPATH=/Users/administrator/Documents/PolicyEngine/microplex-us/src PYTHONDONTWRITEBYTECODE=1 /Users/administrator/Documents/PolicyEngine/worktrees/microplex-us/fix-pe-rebuild-smoke-issues/.venv/bin/python -m pytest -q -p no:cacheprovider tests/pipelines/test_artifacts.py::test_source_provider_versioned_build_initializes_live_stage_writer

A full tests/pipelines/test_versioned_artifacts.py run passed before the final non-directory guard was added; after the guard, the focused allocator tests above cover the changed behavior.

[MaxGhenis]

Closing rather than merging — same situation as #155. microplex-us went spec-only in #261 and is archived; pipelines/versioned_artifacts.py was deleted in that conversion, which is why this is CONFLICTING, and the passing CI is stale. The build/checkpoint logic moved to populace's us_fiscal_refresh builder, which is a different checkpoint model, so this allocation relaxation doesn't carry over — #179 needs re-triage there if it still happens.

On the change itself: the logic was careful and the safety property holds. Reuse only triggers when every entry under the version dir resolves inside a configured checkpoint tree, so a stray real artifact still raises FileExistsError, and it fails closed when checkpoints aren't configured. The stage_runtime_writer pass-through was a correct consistency fix. One coupling worth knowing if it's ever ported: the checkpoint-root helper hardcodes the two checkpoint config keys, so a third checkpoint stage would fail allocation until added — safe, but worth a comment in any successor.